So I wrote a little web app that allows a user to move their user data, like settings and subscribed/banned communities, from one account/instance to another.
It runs completely client-side, but is hosted on GitHub for the moment. Maybe it’ll be of some use!
The code is completely written in JavaScript, so all the code is readable if you look at the source, which is also available on the GitHub page. https://stablenarwhal.github.io/LemmyInstanceMover/js/script.js
It looks like it uses a Lemmy API endpoint to transfer account settings.